Polyisocyanate mixtures containing alkoxysilane groups have been known for some considerable time. Products of this kind, which as well as the isocyanate group contain a second reactive structure, in other words a structure capable of crosslinking, have been used in the past in various polyurethane systems and polyurethane applications with the aim of obtaining specific properties, such as, for example, to improve the adhesion, chemical resistance or scratch resistance of coatings.
By way of example, WO 03/054049 describes isocyanate-functional silanes, prepared from aliphatic or cycloaliphatic polyisocyanates of low monomer content and from secondary aminopropyltrimethoxysilanes, as adhesion promoters for polyurethane hotmelt adhesives.
According to the teaching of JP-A 2005-015644 as well, polyisocyanates or isocyanate prepolymers that are modified with N substituted, i.e., secondary, aminopropylalkoxysilanes can be used to improve the adhesion of adhesives and sealants.
EP-B 0 994 139 claims reaction products of aliphatic and/or cycloaliphatic polyisocyanates with substoichiometric amounts of alkoxysilane-functional aspartic esters, as described in EP 0 596 360 as coreactants for isocyanate-functional compounds, and optionally with polyethylene oxide polyether alcohols, as binders for one-component, moisture-crosslinking coatings, adhesives or sealants featuring accelerated curing.
Reaction products of aliphatic and/or cycloaliphatic polyisocyanates with substoichiometric amounts of alkoxysilane-functional aspartic esters or secondary aminoalkylsilanes are also described in WO 02/058569 as crosslinker components for two-component polyurethane adhesion primers.
EP-B 0 872 499 describes aqueous, two-component polyurethane coating materials which as a crosslinker component comprise compounds containing isocyanate groups and alkoxysilyl groups. The use of these specific polyisocyanates leads to coatings having improved water resistance in conjunction with high gloss.
Hydrophilically modified polyisocyanates containing alkoxysilane groups, which are therefore easier to emulsify, have likewise already been identified as crosslinker components for aqueous 2K (two-component) coatings dispersions and adhesives dispersions (e.g. EP-A 0 949 284).
Particular significance is possessed by polyisocyanates containing silane groups for the production of scratch-resistant coatings.
To improve the scratch resistance of solvent borne thermosetting 2K automotive PU clearcoat and topcoat materials, there have been suggestions of reaction products of aliphatic and/or cycloaliphatic polyisocyanates with N,N-bis(trialkoxysilylpropyl)amines as a crosslinker component (EP-A 1 273 640).
A number of further publications likewise describe coating compositions for production of weathering-stable, scratch-resistant clearcoat layers for OEM automotive production line finishing and automotive refinishing, in which crosslinker components used are reaction products of aliphatic and/or cycloaliphatic polyisocyanates with molar deficiency amounts of alkoxysilanes that are reactive toward isocyanate groups. Suitable reactive alkoxysilanes mentioned are especially specific mixtures of bis(alkoxysilylalkyl)amines with N-alkylmono(alkoxysilylalkyl)amines (e.g. WO 2008/074489, WO 2008/074490, WO 2010/149236 and WO 2014/086530), mercaptoalkylalkoxysilanes and/or secondary alkoxysilylalkylamines (e.g. WO 2009/156148).
By far the majority of the known polyisocyanates that contain silane groups and are commercially available on the market derive from polyisocyanates, especially polyisocyanurate polyisocyanates, of 1,6-diisocyanatohexane (hexamethylene diisocyanate, HDI).
Alkoxysilane-modified HDI polyisocyanates have good compatibility with a multitude of different paint binders and lead to coatings of high gloss and high transparency that are especially notable for very good resistance to scratching because of their self-healing properties. Under “reflow” conditions, i.e. brief heating to temperatures above the glass transition temperature (Tg), fine scratches, once they have been inflicted on such paints, heal to such an extent that gloss is restored to close to the starting gloss of the unscratched paint film.
For a number of applications, for example for automotive refinishing and especially OEM automotive production line finishing, there is a constant demand for coating compositions having further-improved scratch resistance. Of particular interest here are 2K PU coating materials having “true” scratch resistance, i.e. those where scratching leads only to a very small loss of gloss per se and there is no reliance on a reflow effect with regard to maintenance of optical quality.